1. Field of the Invention
The present invention relates generally to thermocouples and, more particularly, to a system and method for forming a thermocouple circuit exhibiting reduced levels of thermocouple drift.
2. Technical Background
Conventional thermocouples are typically manufactured by physically coupling two thermoelectric elements of dissimilar composition to one another. The thermoelectric elements are typically wires of dissimilar metal, wherein ends of the wires are twisted together or otherwise joined. A temperature difference between the junction of the coupled wires and remote portions of the wires will develop a voltage between the opposite ends of the wires. A voltage measuring device (e.g. voltmeter) may then be coupled into the circuit to detect the voltage, and the voltage correlated with a temperature. Thermocouple performance and accuracy is dependent upon uniformity of both physical and chemical properties along the entire length of the circuit, and in particular the thermoelectric elements (e.g. wires). When thermoelectric element materials are produced, careful steps are taken to assure that this uniformity (or homogeneity) is achieved. However, in use, diffusion or migration of chemical species within the thermoelectric materials can result in a change in chemical composition of the thermoelectric elements, thus resulting in drift or inaccuracy of the thermocouple performance.
For example, an exemplary conventional type B thermocouple, such as that shown in FIG. 1, is comprised of a first wire 10 having a chemical composition that is an alloy of 94% platinum and 6% rhodium. A second wire 20, physically coupled to the first wire at the hot junction 30 is comprised of an alloy of 70% platinum and 30% rhodium. Hot junction 30 is shown coupled to an electrically conductive substrate 40 comprising 80% platinum and 20% rhodium. A voltage develops across the ends of the wires opposite the hot junction as a result of the familiar Seebeck effect, which voltage is primarily a result of the temperature gradient along the length of the wires. The voltage may be read with a conventional measuring device, such as a voltmeter, and the voltage correlated to a temperature of the substrate.
Analysis of data from such a conventional system indicates that this exemplified thermocouple design loses calibration while in operation at high temperature, such as a temperature of approximately 1650° C., with an average net rate of drift value of up to approximately −2.7° C. over 30 days due in large part to diffusion of rhodium between the wires and the substrate. That is, the rhodium concentration between the co-joined elements—the wires and the substrate—begins to equilibrate. To that end, it will be appreciated that when processes and systems are under temperature control, and a thermocouple design that is employed to facilitate the temperature control results in a drift in registered temperature, the perceived change in temperature due to the drift can result in an forced change in the actual temperature that is unwarranted, which can lead to a degradation in the operation of the process or system.